J Mol Model (2014) 20:2268 DOI 10.1007/s00894-014-2268-7

ORIGINAL PAPER

Interaction of β-cyclodextrin as catalyst with acetophenone in asymmetric reaction: a theoretical survey Yali Wan & Xueye Wang & Na Liu

Received: 2 March 2014 / Accepted: 23 April 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The asymmetric reduction of acetophenone with sodium borohydride in the presence of β-cyclodextrin (βCD) as catalyst can improve selectivity and yield. The interaction between acetophenone and β-CD plays an important role for the reduction of acetophenone. This work studies the reaction of acetophenone in the presence of β-CD using density functional theory (DFT) method. Energy is investigated to find out the lowest energy of two possible complexation models. The geometrical structure confirms that acetophenone inserts into the cavity mainly from the secondary hydroxyl side. Hydrogen bonds are researched on the basis of natural bonding orbital (NBO) analysis, the results confirm the donor–acceptor interactions of complex. Mülliken charge and frontier orbital are employed for revealing the electronic transfer. In addition, 13 C nuclear magnetic resonance (13CNMR) spectroscopy shows that the active site concentrates on the carbon atom of carbonyl group. The probable catalytic mechanism of β-CD is discussed in terms of the calculated parameters. Keywords Acetophenone . Catalyst . Cyclodextrin . Density functional theory (DFT) . Inclusion complex

Introduction Cyclodextrins (CDs) are the cyclic oligomers of six, seven, or eight glucose units linked by α-1, 4 bonds, which are called α-, β-, or γ-CD, respectively [1–4] (Fig. 1). Their shape is a conical cylinder with hydrophobic interior and hydrophilic Y. Wan : X. Wang (*) : N. Liu Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, People’s Republic of China e-mail: [email protected]

exterior [3, 5]. The wider upper rim is surrounded by the whole of the secondary hydroxyl groups, while the narrower one lines with the primary hydroxyl groups [4]. Owing to their particular structures, they have the ability to offer a suitable microenvironment for asymmetric reaction [5]. The microenvironment can play an important role for changing the physicochemical properties of the guest [6, 7]. Thus, a lot of work using CD as catalyst was reported, such as the synthesis of 2phenylbenzimidazole [8] and azide-alkyne cycloaddition reaction [9], and several reviews about CD catalyst have been reported [10–13]. Interest in the catalyst of CD is increasing for its excellent property. Currently, the experimental method is usually utilized, however, there is few theoretical study works on the CD as catalyst. Many of aromatic ketone asymmetric reductions have been studied in the experiment [14–16]. The reduction of acetophenone (Fig. 2) has been reported, it can happen without solvent and the insoluble inclusion complex is advantageous to the reaction [17]. Thus, theoretical study on the first step of the reaction in gas phase is viable to clarify the effect of CD catalyst. The highly efficiency of CD catalyst for the reduction of acetophenone has been thoroughly studied experimentally. To the best of our knowledge, however, the interaction of host-guest complex between CD and acetophenone in this reaction is still unknown. In this paper, computational method is performed to describe the interaction of β-CD as catalyst with acetophenone. The internal cavity of β-CD is about 8 Å deep and 6.0–6.4 Å in diameter and it possesses a relatively low polarity [18]. Therefore, β-CD has a more suitable cavity for accommodating guest molecules among CDs [19], so β-CD is perfect to be used as the host. The orientation and position of the substrate in the host cavity are investigated for ensuring the minimum conformation. What is more, hydrogen bonds, electronic transfer, and 13C nuclear magnetic resonance (13CNMR) are discussed in the work. Furthermore, the mechanism of CD as

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OH

n=6α-CD n=7β-CD n=8γ-CD

O O

n HO

which is faster than other ways [21]. The isolated β-CD and acetophenone are optimized using DFT B3LYP/6-31G(d,p) method. The inclusion complexes of each step are optimized by PM3 without any restriction. The precise optimization of inclusion complexes are performed with the DFT B3LYP/631G(d,p) method which is based on the results of the preliminary PM3 calculations and then the hydrogen bonds, frontier orbital, and 13CNMR are performed with the basis sets of 631+G(d,p), 6-31G(d,p) and 6-311+G(2d,p), respectively.

OH

Results and discussion Fig. 1 The structure of cyclodextrin

Energy

catalyst in asymmetric reduction is clarified on the basis of these results.

The energy of the preliminary optimization using PM3 is calculated to find the minimum. Figure 4 shows the relationship between the energy of the inclusion complex and position for two possible complexation models. As shown in Fig. 4, multiple local minima can be found for the entire process of inclusion. There are two local minima in model a and two local minima in model b. Only the global minimum of the two models are further refined and they are named complex a (CA) and complex b (CB), respectively. Table 1 lists the interaction energy of the global minimum for the two models. The negative changes of interaction energy demonstrate that the inclusion process is energetically favorable. The interaction energies for CA and CB are −47.78 kJ mol−1 and −26.26 kJ mol−1, respectively. The complex is more stable with the increasing negative of the interaction energy. Therefore, the parameters recorded in Table 1 indicate that the CA is more favorable and stable than CB. This result is in accordance with NMR spectroscopic studies on the inclusion complexes with aromatic guests in experiment [17].

Computational section Computational model The initial geometry of β-CD is constructed in Chem3D Ulter (Version8.0) according to the crystal structure, and the structure of acetophenone is built using Chemdraw and Chem3D Ulter. The host-guest complexes of β-CD and acetophenone are constructed by manually in GaussView making the acetophenone into β-CD cavity through two possible ways (Fig. 3). The glycosidic oxygen atoms of β-CD are placed on XY plane and their center is defined as the same with the coordinate system, meanwhile, the center of the guest is placed on Z-axis [4, 18]. The relative position between host and guest is measured by the distance between the centers of acetophenone and β-CD. The guest molecule passes through the host cavity along the Z-axis from 5 to −5 Å with a stepwise 1 Å and revolves from 0 to 360° around the Z-axis at 30° intervals. Computational method All the calculations are performed using the Gaussian03 software package [20]. The method of PM3 proves to be a powerful tool as the first step for large molecular complex, Fig. 2 Asymmetric reduction pathway of acetophenone with βCD as catalyst

H3C

Geometrical structure The relative conformational energy obtained from B3LYP is more accurate for DFT calculation [22]. Thus the conformations of CA and CB are optimized with B3LYP. Figure 5 shows the top and side view of CA and CB. The carbonyl group in CB is almost completely surrounded by the β-CD and part of phenyl group is out of the cavity. OH

O H3C

β CD step 1

insolube inclusion complex

NaBH4 step 2

∗C

H

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Fig. 3 The two possible complexation models of acetophenone into β-CD

O Z

CH3

model a

Y

X

O

CH3

Z

model b

Y

However, the phenyl group in CA is completely in the cavity, the carbonyl group is close to the secondary hydroxyl rim and exposed to the hydrophilic part. The geometry of acetophenone molecular complex whether in the cavity of CA or CB is tilted rather than vertical. What is more, the C1-

C3-C12-O13 dihedral angles of acetophenone in the CD cavity are −11.15° (CA) and 5.77° (CB), respectively, which is nearly −0° in the free acetophenone. The distances of glycosidic oxygens in cavity have some slight changes. Therefore, both acetophenone and β-CD have a litter deformation and change in complex.

Table 1 Energy parameters of two models calculated by DFT B3LYP/631G(d, p) Species

Model a

Model b

Ecoma(kJ mol−1) ΔEb (kJ mol−1) ΔEdb (kJ mol−1)

−12,235,605.05 −47.78 2.63

−12,235,074.17 −26.26 0.612

a: Ecom: the HF energy of the inclusion complex from optimization b: ΔE is the interaction energy calculated according to ΔE = Ecom−Ehost−Eguest; ΔEd is the deformation energy calculated according to ΔEd =E(guestsp)−Eguest Ehost, Eguest: the HF energy of the host and guest from optimization Fig. 4 Graphic diagram of the energy for the inclusion complex of acetophenone into β-CD at different positions

E(guest-sp): the single point energy of guest in the optimized inclusion complex

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Fig. 5 B3LYP optimized geometries a top view of CA b side view of CA c top view of CB d side view of CB

a

b

c This alteration in structure demonstrates positive interactions for these complexes, which can cause mutual interactions and increase the number of hydrogen bonds between the hydroxyl groups of β-CD and carbonyl group. The deformation energy of the guest recorded in Table 1 demonstrates that the guest in CA needs more deformation energy than in CB. The value of the deformation energy is an important sign for the driving force toward complexation and the increasing value can lead to a more favorable inclusion [23]. Therefore, the CA is the minimum conformation. Hydrogen bonds The geometrical structure analysis supports that the substrate in CA is more flexible than that in CB. Thus, the hydrogen bonds of CA are expected to be stronger than CB which is due to the flexibility of carbonyl group in β-CD cavity, and a lot of details about hydrogen bonds are obtained from the natural bond orbital (NBO). Table 2 contains amounts of parameters which can be utilized to describe the X–H⋅⋅⋅Y hydrogen bonds systems. The second-order Møller-Plesset perturbation stabilization energy E(2) which is in relation to the donor-acceptor delocalization [24] is performed to measure the strength of hydrogen bonds. The total of E(2) indicates the hydrogen bonds strength between β-CD and acetophenone. From Table 2, it can be noted that the sum in CA is greater than in CB and the shorter

d O⋅⋅⋅H distance and larger angle of X–H⋅⋅⋅Y in CA are also corresponding to the larger E(2). The value of E(2) in CA reveals that more hydrogen bonds form from β-CD to acetophenone, but these lone pairs are mainly delocalized to the anti-bonds of β-CD. This phenomenon is decided by the two lone pairs on oxygen atom (O158) of carbonyl group. They are delocalized to O105-H134 and C20-H67 which result in strong stabilization energy. Additionally, the calculated data indicate that there are two classical hydrogen bonds and three weak hydrogen bonds in CA. Thus, it can imply that the hydrogen bonds play a significant role in CA, and the hydrogen bonds between oxygen atom (O158) and secondary hydroxyl make a great contribution to driving the acetophenone molecular into the cavity of β-CD and keeping CA stable. Electronic transfer The Mülliken charge analyses for isolate acetophenone and CA are shown in Table 3. The sum charges of acetophenone change from 0 to 0.466 upon complexation, and the charge transfers mainly focus on the carbon atom of carbonyl group. This indicates the acetophenone molecular with positive charges in the complex. In other words, the guest as Lewis base donates electron upon complexation and the total charges of all atoms are certainly zero, so β-CD as Lewis acid in CA must accept the electron. The frontier orbital which can reflect

J Mol Model (2014) 20:2268 Table 2 Bond lengths (Å), angles (°), and stabilization energy E(2) (kcal mol−1) of hydrogen bonds in CA and CB at B3LYP/631+G(d,p) level

a: Cross hydrogen bond with two coordination

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CA From β-CD to acetophenone Hydrogen bond RO…H C159-H162…O96 2.37 C156-H161…O113 2.71 C154-H160…O118 2.70

CB

θ 171.88 173.46 163.88

E(2) 3.58a 0.68 0.66

Hydrogen bond C151-H155…O121

RO…H 2.96

θ 135.56

E(2) 0.29

From acetophenone to β-CD C105-H134…O158 1.82 171.69 C20-H67…O158 2.47 128.02 Sum of stabilization energies 22.3 2.85

15.8a 1.54a

C22-H69…O160 C20-H67…O160

2.65 2.61

148.94 151.02

1.12a 1.44a

the electronic structures are investigated for more about the electronic transfer. Highest occupied molecular orbital (HOMO) and lowest virtual molecular orbital (LUMO) of free acetophenone and CA are shown in Fig. 6. The LUMO of free guest and CA reveal that it remains nearly unchanged upon inclusion for acetophenone. The energy gap is a significant sign for chemical activity and small value of energy gap means less stability [25, 26]. The energy gaps of isolated guest and CA are 5.225 eV and 4.680 eV, respectively. Thus, acetophenone in the cavity of β-CD is easy to react with NaBH4. From the frontier orbital of CA, it should be noted that HOMO is mainly spread around β-CD, while the LUMO almost focuses on acetophenone. The HOMO represents the ability to donate an electron while the LUMO as an electron acceptor represents the ability to obtain an electron [27]. That is to say, β-CD is the nucleophilic region and the electrophilic region is dominated by acetophenone. This conclusion is in good agreement with the result of Mülliken charge.

(GIAO) method [28] and tetra methyl silane (TMS) as internal reference [29]. The chemical shifts of acetophenone have some slight difference with the experimental result [30]. Consequently, the calculated NMR is reasonable for qualitative research. C1 and C5 are on the meta, C2 and C4 are on the ortho, while C6 is on the para of phenyl group. Due to the deformation of the structure, there are some differences in the values of carbon atoms in the same position of phenyl group. As is known, low shielded electrons mean upfield and vice versa. From the information in Table 4, it is easy to note that all atoms except for C2 and C3 in complex have a high chemical shift in contrast to the free. It represents that most carbon atoms possess more positive charges and high electropositive in the presence of β-CD. This result of chemical shift is in good correlation with electronic transfer. In addition, the C7 atom in carbonyl group shows high chemical shift compared with others. It indicates that more positive charge concentrates on the carbon atom of carbonyl group in CD as the catalyst.

13

Catalytic mechanism

13

Based on the conclusions of energy, geometrical structure, hydrogen bonds, electronic transfer, and 13CNMR, the mechanism of β-CD as catalyst with acetophenone in asymmetric reaction is discussed. In the process of reduction, NaBH4 can

CNMR

CNMR spectrum is sensitive to reflect alteration in the chemical environment of carbon atom. Thus, the 13CNMR chemical shifts of acetophenone in the absence and presence of β-CD are investigated with gauge-including atomic orbital

Table 3 Mülliken charge analysis for isolate acetophenone and acetophenone in CA at B3LYP/631+G d,p) level

Atom

Isolate acetophenone

acetophenone in CA

Total charges of carbon atoms in phenyl group Total charges of hydrogen atoms in phenyl group Charge of carbon atom in carbonyl group Charge of oxygen atom in carbonyl group Charge of carbon atom close to carbonyl group Total charges of hydrogen atoms close to carbonyl group Total charges

−0.456 0.591 0.283 −0.411 −0.551 0.544 0

−0.169 0.614 0.622 −0.385 −0.778 0.562 0.466

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Fig. 6 HOMO and LUMO orbitals of free acetophenone and CA

produce hydrogen anion to combine with the carbon atom of carbonyl group. In the presence of β-CD, the acetophenone enters into the cavity of β-CD forming hydrogen bonds between oxygen atom (O158) and secondary hydroxyl, then the electron density redistributes between β-CD and acetophenone. The electrophilic region is prevailing on the acetophenone molecular and mainly focuses on the carbon atom of carbonyl group. Thus, the activity of carbon atom in carbonyl group of acetophenone reacting with the hydrogen anion is increased. If the stereo environment of two sides in carbonyl group is different, the reduction can produce two different products which is an asymmetric reaction. The phenyl group inserts into the cavity mainly from the secondary hydroxyl side and the carbonyl group is positioned on the wider rim in complex. The asymmetric environment of wider rim is larger than the other rim which is in favor of

Table 4 Variation in 13 CNMR chemical shift (δa) of acetophenone upon complexation

a

δ=σ TMS −σ.

b

Δδ=δcomplex− δfree

δfree

δcomplex

Δδb

C1 C5 C2 C4

133.86 132.96 134.81 134.74

133.87 135.92 134.77 137.43

0.01 2.96 −0.04 2.69

C3 C6 C7 C8

142.59 138.67 202.12 27.62

141.92 141.16 212.06 29.12

−0.67 2.49 9.94 1.50

Free HOMO

Free LUMO

CA HOMO

CA LUMO

asymmetric reaction. The acetophenone is tilted in the cavity. Therefore, the attack of hydrogen anion happens more easily on one face of the carbonyl group than the other. As a result the CD as catalyst can be used to improve the enantioselectivity. According to the mechanism of acetophenone with NaBH4, it can be concluded that βCD catalyst improves the activity of acetophenone and increases the enantioselectivity by the way of molecular complex.

Conclusions The optimization results show that the most stable complex form in model a, that is, phenyl group inserts into the cavity mainly from the secondary hydroxyl side and the carbonyl group is positioned on the wider rim. The hydrogen bonds of the global minima in model a and model b suggest that the complex is driven by hydrogen bonds. The electronic transfer reflects that acetophenone gets more positive charges in complex, β-CD is the nucleophilic region, and the electrophilic region is dominated by acetophenone. 13CNMR spectrum shows that a lot of positive charges concentrate on the carbon atom of carbonyl group. The CD catalyst can improve the activity of acetophenone and increase the enantioselectivity by the way of molecular complex. Acknowledgments The authors wish to acknowledge the financial supports from the Scientific Research Fund of Hunan Provincial Education Department (No. 201104) for the research work.

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Interaction of β-cyclodextrin as catalyst with acetophenone in asymmetric reaction: a theoretical survey.

The asymmetric reduction of acetophenone with sodium borohydride in the presence of β-cyclodextrin (β-CD) as catalyst can improve selectivity and yiel...
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